Two selenocysteine tRNAs have been isolated and characterized from mammalian cells in this laboratory. They read the termination codon, UGA, in protein synthesis and thus donate selenocysteine to the growing polypeptide chain in response to UGA codons. The biosynthesis of selenocysteine occurs on the tRNA isoacceptors and its pathway is tRNA[Ser)Sec ---> seryl-tRNA[Ser]Sec --- > phosphoseryl-tRNA[Ser]Sec --- > selenocysteyl-tRNA[Ser]Sec. Both isoacceptors arise from a single copy gene in each of the organisms from which the gene has been isolated and sequenced. The organisms include human, rabbit, chicken, Xenopus, Drosophila and C. elegans. Transcription of the gene is regulated by several upstream regions. The internal control regions of the gene appear to dictate the site of initiation of transcription, as well as playing a role in processing at the 3' end of the primary transcript. The two isoacceptor gene products differ from each other by five pyrimidine transitions which occur in the 5' half of the molecule. Therefore, one isoacceptor is likely an edited version of the other. Addition of selenium to the media of mammalian cells in culture induces editing of the selenocysteine isoacceptors. We have also shown that the UGA codon in glutathione peroxidase codes for both termination and for selenocysteine, demonstrating the dual role of this codon in the genetic code. In addition, a selenocysteine TRNA has been found in yeast and in algae demonstrating that UGA is used for both termination and selenocysteine within the universal genetic code.